A few years ago the W3C (World Wide Web Consortium) created a mechanism for browsers to inform web servers that they would not like to be tracked. This is accomplished by including a DNT (Do Not Track) header with web requests.
The DNT header doesn't really provide any privacy because most web servers ignore it. For example, Yahoo, Google, Microsoft, and Facebook all ignore at least some DNT headers. Beginning with version 3.8, Privacy Browser no longer has the option to send a DNT header.
Privacy Browser removes tracking IDs and AMP (Accelerated Mobile Pages) redirects from URL queries. There is a blog post that is updated with the current list of modifications. URL modification can be turned off in the settings if it is causing issues.
Google programmed Android’s WebView to send an X-Requested-With header with every request.
The value of the X-Requested-With header is set to the application ID, which in the case of Privacy Browser is com.stoutner.privacybrowser.standard.
Currently, it isn't possible to remove this header, but the value can be changed. By default, Privacy Browser sends a null (empty) value for the X-Requested-With header.
Even though the spec allows for null header values, some web servers don't like them, so Privacy Browser has the option to revert to the default behavior of sending the app ID.
Because this setting is cached when a URL is first loaded, and isn't changed when reloading a page or navigating history,
changes to this setting may not be applied until Privacy Browser is restarted.
There is a blog post with additional information.
The X-Requested-With header will be completely removed in the 4.x series with the release of Privacy WebView.